1672 had been a catastrophe for the Dutch. Louis XIV crossed the Rhine in June, the bishop-prince of Munster joined him from the north, and by autumn most of the Republic west of the Hollandic Water Line was under French boots. At sea, Michiel de Ruyter saved his country with three improbable defensive victories at Solebay, Schooneveld, and Texel, but on land the Dutch survived only by flooding their own farms. The young stadtholder William III, twenty-two years old and barely seasoned, spent the winter rebuilding alliances. By September 1673 he had what 1672 had lacked: Imperial troops under Raimondo Montecuccoli marching west from the Main, and a Spanish corps moving up from the Netherlands. The plan was not to fight Louis directly. The plan was to cut his supply line.

William's army crossed the Meuse near Venlo in October, then drove south through the lands of the Archbishop-Elector of Cologne. The march was deliberately punitive. Dutch soldiers torched towns the way French cavalry had torched theirs the year before, and the Cologne territories that had cheered Louis on in 1672 now watched their barns burn. At Rheinbach, where townspeople killed two Dutch officers, William ordered the place stormed and every armed defender executed. The mayor was hanged from his own city gate with the town keys still around his neck and his sword still in his hand. It was not the kind of war that produced romantic engravings. It was the kind that produced surrenders elsewhere, which was the point.

Bonn was the Archbishop-Elector's seat and the only fortified bridgehead the French still controlled on the middle Rhine. Its walls were modern by 1673 standards, with nine bastioned fronts and dry ditches, but its riverside defenses were just a wall and the Rhine, and no bridgehead protected the eastern bank. Maximilian Henry of Bavaria, the archbishop, had already fled to Cologne. Effective command inside the city fell to a French colonel named Reveillon with two French regiments and somewhere between 1,700 and 2,000 men. Around the city, the allies arranged themselves in three camps. William III set his headquarters at Rheindorf to the north. The Spanish general Pedro de Acuna y Meneses, Marquess of Assentar, took Kessenich to the southwest. Montecuccoli pitched his tent on the Godesberg, the same volcanic cone where, ninety years earlier, Bavarian sappers had blown a fortress apart in the Cologne War. The geography insisted on irony.

Siege works opened on 5 November. Dutch sappers pushed trenches toward the Cologne Gate from the north, Imperial sappers approached from the south, and a pontoon bridge linked both banks of the Rhine. The garrison tried sorties. On the night of 6 November they slipped out and burned the beacons guiding Imperial trench parties, which briefly threw the southern approach into chaos. A French cavalry officer named Saint Silvestre managed to slip a small detachment into the city at dawn, disguised as Lorraine cavalry. None of it mattered enough. Allied batteries opened on 8 November and within three days had wrecked the ravelin in front of the Cologne Gate. Dutch infantry stormed the broken position on the night of 11 November, taking heavy casualties but holding the ground. Reveillon prepared to fight for the main wall. His Cologne officers and the townspeople, by then, were ready to be done.

Negotiations opened on 12 November. The garrison marched out on the 13th, allowed to withdraw to Neuss with the honors of war. Casualties on each side ran around four to five hundred. None of those numbers explain the importance of what happened next. As soon as Bonn fell, Louis XIV's foothold in Holland became geographically untenable. His communications with France had to thread the Rhine, and now Imperial cannon controlled the Rhine south of Cologne. By the end of November, French garrisons were abandoning Utrecht, Gelderland, and Overijssel. The bishop of Munster pulled out of Meppel and Steenwijk. Only Grave and Maastricht held on, and both surrendered within the year. Maximilian Henry signed peace with the Dutch in spring 1674. The Archbishop-Elector who had bet on the Sun King discovered that the Sun King would not be coming back.

Bonn would be besieged twice more, in 1689 and 1703, each time more elaborately and each time with heavier casualties. The 1673 siege is the quietest of the three in popular memory, partly because nothing in it was tactically novel and partly because William III, the soldier who set it all in motion, would become better known fifteen years later as William III of England. But for ten days on the Rhine in November 1673, a young Dutch stadtholder, an aging Italian field marshal, and a Spanish corps from Brussels turned a regional siege into the lever that ended Louis XIV's first Dutch war. The Sun King would launch other wars and win other campaigns, but he never again came as close to swallowing the Netherlands as he had been in the summer of 1672. The men who took Bonn made sure of it.